1. Field of the Invention
The invention generally relates to electronics, and in particular, to communications circuits.
2. Description of the Related Art
Communications circuits can exhibit certain input characteristics and certain output characteristics. For example, some devices may have current outputs, while others may have voltage outputs. Some devices may have current-driven inputs, whereas other devices may have voltage-driven inputs. Even when the input and output characteristics of devices apparently match, buffering may be used because of non-linearity in the input and/or output characteristics.
For example, a filter can be sensitive to the non-linearity of a load, such as a mixer. A buffer circuit can be used to isolate the non-linearity of the load from the filter, which could otherwise cause intermodulation distortion. Intermodulation distortion is typically an undesirable characteristic, as it can corrupt the spectrum and decrease the number of channels that can be carried by a signal to maintain a particular signal to noise ratio.
Thus, buffering circuits are relatively common in electronic circuits. Some buffering circuits can also convert from voltage to current or from current to voltage. Other buffering circuits simply buffer current or voltage.
FIG. 1 illustrates an example of a conventional buffer circuit 100. The buffer circuit 100 receives currents Iip, Iin as inputs and generates currents Iop, Ion as outputs. The buffer circuit 100 has resistors Ri 102, 104, transistors Q1-Q4, resistors Re 106, 108, and current sources 110, 112. The resistors Ri 102, 104 form a current-to-voltage converter. The transistors Q1-Q4, the resistors Re 106, 108, and the current sources 110, 112 form a voltage-to-current converter.
The current-to-voltage conversion operates as follows. The current through the bases of the transistors Q3, Q4 is negligible compared to the current through the resistors Ri 102, 104. Thus, the input currents Iip, Iin flow through the resistors Ri 102, 104. The voltage drop across the resistors Ri 102, 104 converts the input currents Iip, Iin to the voltages Vip, Vin.
The voltage-to-current conversion operates as follows. A voltage loop from the base of the transistor Q4, to the emitter of the transistor Q4, to the base of the transistor Q2, to the emitter of the transistor Q2, and across the resistor Re 106 is considered. The voltage at the base of the transistor Q4 is the voltage Vip. The base to emitter of the transistor Q4 adds a VBE voltage, and the base to emitter of the transistor Q2 subtracts a VBE voltage. The circuit can be designed such that the two VBE voltages approximately match and thus cancel. The voltage Vip then appears across the resistor Re 106, which then passes a current of the voltage Vip divided by the resistance Re. Assuming that the collector current Iop of the transistor Q2 is about the same as the emitter current of Q2, which flows through the resistor Re 106, then the collector current Iop is approximately equal to the voltage Vip divided by the resistance Re. The approximate relationship between the output current Iop and the input current Iip is expressed in Eq. 1.
                              I          op                =                                            V              op                                      R              e                                =                                                    I                ip                            ⁢                              R                i                                                    R              e                                                          Eq        .                                  ⁢        1            
The currents Iin, Ion and the voltage Vin can be analyzed in the same manner. The current sources 110, 112 provide biasing for operation.